Add paragraph tags to 'Add Existing User' section on 'Add New User' screen in multisite.

props jkudish, topher1kenobe.
fixes #20271.
Built from https://develop.svn.wordpress.org/trunk@28642


git-svn-id: http://core.svn.wordpress.org/trunk@28460 1a063a9b-81f0-0310-95a4-ce76da25c4cd